my prof. talked about trans heterozygous phenotype, but not sure what he meant. as far as i understand, it's used to identify other genes that are involved in a same pathway. but he wrote on his note that "when two genes are in the same pathway, mutants heterozygous for both genes will display a phenotype even though each individual heterozygous mutant does not". can anybody tell me what he meant here?? also, any other outisde info about this trans heterozygous phenotype would be appreciated. thanks.

It looks like you understand everything well. a transheterozygote is an organism whose genotype regarding to genes A & B is: A-/A+, B-/B+.
When you have the same organism which is A+/A+, B+/B- or A+/A-, B+/B+ it's completely normal but the transheterozygote displays some defects.
I just did a presentation on two genes Robo and Slit which mediate midline crossing by axons in the brain. A robo+/- mouse or slit +/- is normal but a robo+/-, slit +/- has defects in crossing of comissural axons. this is a kind of evidence that these genes work in the same pathway, and indeed slit is a ligand for robo.

"When In Danger Or In Doubt Run In Circles Scream And Shout"Lawrence J. Peter